Thursday, November 27, 2008

Astrophotography, how it is done,...

How it is done, or at least how I do it.

The path I have taken has been to attempt to follow the methods of many others. Yahoo has several groups which concern themselves with astrophotography the subscribing to these is a great resource. Most books on astronomy devote at least a chapter on this topic, methods and guidelines described are helpful.

The most motivating materials I have found are at the web sites of other amateur astronomers. I discovered very early, that Hap Griffin was able to produce images beyond belief with equipment not beyond reach. Some astrophotographers use cameras priced >$10k and scopes on mounts in similar ranges. I liked knowing that there was a real possibility of remarkable success and that it was only a matter of time and skill. It is always interesting to put theory into practice.

  • Planning
The first step is to do a bit of planning. I use a program called AstroPlanner for this task, well its not really a "task". This program lets me log objects I have observed, and with it's database of object information I can decide what choice of camera, scope, focal reducer, or barlow will best frame the object. Also since this program will also give me the surface brightness I can guess as to the exposure times I may need. This program also will calculate the best eyepiece given a scope choice. I can sort objects by constellation and spend all night wandering through the heavens. This is a great program to browse through downloaded b&w images.

  • Detailed Field of view and Scope Control
Starry Night Pro is a planetarium package which I use for several key steps. It will show me the exact possible field of view my selected camera / scope will provide

I will have aligned my mount to the night sky and Starry Night will direct my scope to point to whatever object I point and click on. Usually the target will be in the field of view and only minor adjustments will be needed to get the final framing.

Using the AstroPlanner package above and StarryNight, I can move from object to object even though in several cases I can not even see them. Often, objects are not bright enough to see but they can still be photographed.

Once the object is framed then the photography can start.

  • Auto Guiding
Auto guiding is a procedure by which a secondary scope and camera are used to monitor a reference star and discipline the mount to stay trained on that star. In this case, one scope will be used for the object photography and the other for guiding. The choice of which is for what is governed by the needs of the object being photographed. Smaller objects will require the higher magnification of the larger scope. Piggybacking the two scopes makes this all possible.

The program K3CCDTools is used to make the auto guiding happen. I use a small CCD imaging camera connected to the PC through USB to monitor the reference star, the program automatically detects the drift of the reference star and provides equal and opposite correction commands to the mount. This prepares the mount and thus the main scope and camera for it's long multi minute exposures.

  • Finally we get to take a picture
No not yet, we have to focus the camera, and this is a task which is taken for granted in normal daylight photography. Focusing through a scope is a manual operation although this can be automated with the right hardware. The view finder is not a viable option as there is just not enough light to work with. Instead I use a process of; taking a sample image; down load to the PC; examine the quality of the focus; take a guess and adjust the scope's focus; and return. I run this until I have achieved the best I can get. It turns out that this can be done with some help. I use Imageplus for camera control. This program will trigger the camera take the exposure fetch the image and display it. Once I locate a reference star the program will calculate the quality of the focus. It can take 15 images to get to the bets focus the evening will provide. The atmosphere and its turbulence usually limits what can be achieved.

  • OK, so now can take our real exposures.
Exposures are usually done at ISO400 or 800, and exposure times in the range of a few minutes to 15minutes each. In all cases there are advantages to taking several images instead of one long image, and thus 10 or more images will be taken. Altogether, it is easy to see that there can be up to 1hr or more in the total exposure time.

Long exposure times are needed because of the low surface brightness of the object being taken. Clusters, which are comprised of hundreds of thousands of stars in a tight formation, usually always circular do not need long exposures. Large galaxies who's light energy is spread out need longer exposures. Nebulas who's light is int the red end of the spectrum where any consumer camera like the 10D do not have good sensitivity will need extended exposures. Usually the answer is as long as you can tolerate is the best. Increasing the ISO is not usually a good idea as a increase in noise will result. Imageplus, the same program used for focusing is used to execute all of the long bulb exposures, images are stored to the laptop.

Sit and wait, enjoy some coffee from home, in the quiet are what is needed now.

Remember the auto-guiding that was started needs to run the entire time...

Occasionally I have to keep in mind that with a German equatorial mount may become limited as it can not track past the meridian.
  • Post Processing
Post processing reminds me of the dark room work my older brother would do when he was developing his B&W photography back in university days. My dark room has no chemical and water filled trays, spools, lines and closepins. It is rather a comfortable den with a good PC, a nice calm light in the corner, and some tunes to listen too.

Postprocessing starts with looking at each of the individual exposures.

Exposures with flaws like, someone kicked the tripod and made the exposure wonky are marked for discard. Ones with flashing airplanes streaking thought the frame are gently put into the trash. (these are all good reasons for multiple exposures) Below are two samples.

Once all of the frames are processed into one image the final result is achieved. In some cases it is like gold and others are clearly in need of another try. The top image pair, unprocessed above and result below are of Messier Object M42.


The Orion Nebula Messier 42 (M42, NGC 1976) is the brightest starforming, and the brightest diffuse nebula in the sky, and also one of the brightest deepsky objects at all. Shining with the brightness of a star of 4th magnitude, it visible to the naked eye under moderately good conditions, and rewarding in telescopes of every size, from the smallest glasses to the greatest Earth-bound observatories as well as outer-space observatories like the Hubble Space Telescope. It is also a big object in the sky, extending to over 1 degree in diameter, thus covering more than four times the area of the Full Moon.

The lower pair are of Messier Object 51. M51 was one of Charles Messier's original discoveries: He discovered it on October 13, 1773, when observing a comet, and described it as a "very faint nebula, without stars" which is difficult to see. M51 is the dominating member of a small group of galaxies, which also contains M63 and a number of fainter galaxies. As it is about 37 million light years distant and so conspicuous, it is actually a big and luminous galaxy. For the amateur, M51 is easy and a showpiece if the sky is dark, but is quite sensitive for light pollution which easily makes it fade in the background. Under very good conditions, even suggestions of its spiral arms can be glanced with telescopes starting from 4-inch. Low magnification is best for viewing this pair. This object is 1/6 of a degree across or about 2mm at arms length,... very small.

These two examples are near the extremes of what I can photograph. From the large and bright to the very small and faint. One thing is constant, the methods are the same, so this is how it is done, or at least this is the way I do it.


Brad said...

I stumbled upon your blog entry on Astrophotography and your the screen capture you took showing Starry Night's interface caught my eye. How did you get Starry Night to outline the Deep Space Objects like Orion Nebula, Horsehead, and even the ones without pictures like the Witchhead Nebula? On my version (Digital Download 6.2), it only circles the Deep Space Objects and then really only those with pictures. I wouldn't think that a function so basic would be left out of the Digital download version. Thanks.

Adrien Comeau said...

Hi Brad

I use Starry Night Pro Plus version 6.2, in this version I select View-DeepSpace-Nebulae, this enables outlines of items such as Witchhead